Stem cell factor (SCF) is an early-acting cytokine that plays a pivotal role in the regulation of embryonic and adult hematopoiesis. SCF promotes cell survival, proliferation, differentiation, adhesion, and functional activation of cells at multiple levels of the hematopoietic hierarchy. Together with other cytokines such as thrombopoietin and Flt3/Flk-2 Ligand, SCF is commonly used to promote expansion of primitive hematopoietic stem cells and multi-potent progenitor cells in culture (Huang et al.; Kent et al.). In synergy with various growth factors, including IL-2, IL-3, IL-6, IL-7, G-CSF, and erythropoietin, SCF increases proliferation and differentiation of myeloid and erythroid progenitor cells and a subset of lymphoid progenitor cells (Broudy). In the mouse, SCF is essential during fetal gonadal development (Mauduit). It is produced by stromal cells in the fetal liver, bone marrow, and thymus, in the central nervous system, in keratinocytes, and in the gut mucosa, and can function as a chemotactic and chemokinetic factor. SCF exists in two biologically active splice forms: a soluble and a transmembrane isoform. Upon binding to its receptor (c-kit tyrosine kinase receptor; CD117), it activates PI3K, JAK/STAT, and MAPK pathways. SCF and signaling from c-kit has also been reported to play an important role in pigmentation, fertility, vasculogenesis, motility of the gut via c-kit-positive interstitial cells of Cajal, and in the migration of neuronal stem and progenitor cells to sites of injury in the brain (Lennartsson and Ronnstrand).